Yes I can address this. I was intending to talk about grave and serious “circumstances,” not the off topic debate going on right now about “grave matter.”
Denzinger, repeatedly talks about “grave” circumstances, being life threatening circumstances. (Examples: Denzinger 2715, 2795, 3634, 3638).
Here are examples where Denzinger uses the word serious:
Denzinger 1175: With cause it is licit to swear without the intention of swearing, whether the matter be light or serious.
-CONDEMNED
Whoever moves or induces another to bring a serious loss upon a third party is not bound to a restitution of that loss incurred.
-CONDEMNED
Denginger 1547:
“…sentences called ‘ipso facto’ have no other force than that of a serious threat without any actual effect,–false, rash, pernicious, injurious to the power of the Church, erroneous.”
However, serious is used in many contexts, so I would really need to study all the usages, and decrees in Ph.D. level depth to really know the answer.
However, what is clear, is that Church documents that use “serious” today, use it more synonymous with “just.”
Examples: Humanae Vitae referenced above.
Also, on skipping mass, serious is used, but is not just life threatening.
Catechism 2181. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) …
Preliminary conclusion: grave means life threatening, but more recent documents use “serious” to refer to things that are not life threatening. But like I said, I will study this more before forming a strong opinion.